Dimethylcumenes are useful raw materials for the production of xylenols, pesticides, pharmaceuticals, perfumery, heat transfer media, polymers and special solvents. Xylenols are an important class of phenolic substrates useful as raw materials for making phenolic resins having specific thermoelastic properties.
The most common method for the preparation of dimethylcumenes is by the alkylation of xylene carried out in the presence of metal halides, AlCl3, ZnCl2 and inorganic acids HCl, H2SO4 [(i) Journal of Chemical Education, Vol. 70 (6), pages A 152-154, 1993; and (ii) U.S. Pat. No. 5,300,717, which disclose a process for the preparation of dixylylpropane wherein one step comprises the formation of 1,2-dimethyl-4-isopropylbenzene from o-xylene and propene in the presence of conventional Friedel Crafts catalysts such as BF3—H3PO4, AlCl3—CH3NO2). This process suffers from the disadvantage that it is environmentally hazardous, giving rise to problems in terms of handling, safety, corrosion and waste disposal.
Rare earth modified zeolites such as (Nd—Na—Y) are reported as catalyzing the alkylation, including the isopropylation of o-xylene (Dokl Akad. Nauk., Vol. 335 (3), 1994, p. 322-325<CA: 121:182357). In this case, although the isopropylation of o-xylene resulted in the formation of a mixture of dimethylcumenes, alkylation could not be obtained when p-xylene was used as a substrate. The disadvantage of this catalyst system is its inactivity for other xylene isomers such p-xylene. The conversions and selectivity are also low for the desired dimethylcumene isomers. Additionally, this catalyst is less active for isopropylation as compared to tertiary butylation of o-xylene. Solid catalysts such as TiO2—SiO2—Al2O3 (Tisial) and MoO3—SiO2—Al2O3 (Mosial) are also reported for the catalytic alkylation of benzene, toluene and xylene isomers with C2, C3, and C4 aliphatic alcohols (Indian Journal of Chemistry, Vol. 33B, pp. 1053-1061, 1994). However, the yield of desired mono dimethylcumenes is low (ca.50%).
In view of the above-mentioned drawbacks and limitations of the above process, it was found desirable to develop an improved process for the production of dimethylcumene isomers with isopropanol using a solid acid catalyst, either in batch reactor or in fixed bed down flow reactor.